1. Technical Field
The present invention relates to a light-emitting element, an exposure head and an image-forming apparatus.
2. Related Art
An image-forming apparatus, such as a copying machine or a printer, using an electrophotographic method includes an exposure device exposing the external surface of a rotating photoreceptor to form an electrostatic latent image. Among such exposure devices is an exposure head including a plurality of light-emitting elements arranged in the direction of the rotating shaft of the photoreceptor (for example, JP-A-2-4546).
For example, the exposure head disclosed in JP-A-2-4546 has a structure in which LED array chips, each including a plurality of light-emitting elements (LEDs) are arranged in one direction.
It is however difficult to increase the resolution of an image-forming apparatus including the exposure head of the above patent document because only a relatively small number of inorganic LEDs can be arranged in a unit area.
Accordingly, it is proposed that organic electroluminescent elements (organic EL elements) be used as the light-emitting elements of the exposure head.
An organic EL element includes an anode, a cathode and a luminescent layer between these electrodes. In this type of light-emitting element, electrons and holes are injected to the luminescent layer respectively from the cathode and the anode by applying an electric field between the cathode and the anode, and the electrons and the holes are recombined to each other to form excitons in the luminescent layer. When the excitons return to the ground state, the energy is emitted as light.
For example, for use in an exposure head of light having a peak wavelength of 600 nm, the light-emitting element preferably emits light having an intensity of 20,000 cd/m2 or more, and has a lifetime (time period for which the luminance is reduced by 10%) of 1000 hours or more.
For example, Japanese Patent No. 4024009 discloses a highly efficient, long-life organic EL element emitting red light including a luminescent layer containing a naphthacene derivative and a diindenoperylene derivative.
However, the lifetime of this organic EL element is not sufficient to satisfy the characteristics required for the exposure head.